In electrostatic image reproduction, it is known to print two or more superimposed colors on a substrate by separate operations. Printing in two colors is useful, for example, when it is desired to highlight particular sections of documents such as in accounts, where material may be typewritten in two colors, in stationery with two printed colors and so forth. Printing (or copying) in two or more colors generally requires a process which incorporates two passes as well as accurate registration of the two or more superimposed colors.
In U.S. Pat. No. 4,068,938 (Robertson), there is described a process in which a two color plus background copy is obtained from a two color plus background original. According to Robertson's concept, the pairs of colors are chosen according to the response characteristics of the photoconductive surface on which the electrostatic latent image is formed. Thus the electrostatic latent image is characterized by a high level of potential corresponding to locally uniform areas of one of the colors, an intermediate level of potential corresponding to locally uniform areas of the second color, and a low level of potential corresponding with locally uniform areas of the background.
There are two steps in the development of the image in this process: (1) particles of one color are attracted to areas of the electrostatic latent image of high potential and (2) particles of a second color are attracted to areas of low potential. Thus, in the resultant copy, the second color of the original is not reproduced per se but is represented by the color of the copy sheet which can of course be chosen to correspond to the second color of the original. It should be noted that this process depends in practice on the provision of different inks in separate baths so as to develop in separate steps two different areas of the electrostatic latent image.
In contrast to Robertson, U.S. Pat. No. 4,264,185 (Ohta) describes a process in which areas of an electrostatic latent image corresponding to two colors in an original document are developed (rather than one color plus background). Ohta's process applies toners of different colors and polarities to the latent image, in separate steps. In order to repel other, undesired toner particles from one development unit, a bias voltage of opposite polarity is applied to the latter unit, and vice-versa.
Ohta's process may use separate baths of the two toners, or a single partitioned bath with liquid communication between the compartments. Even where a single partitioned bath is used, the latent image is developed in two distinct steps. It is unclear from the disclosure in this patent how the question of clean-up of the background is to be dealt with, if at all.
Ohta's process is based on the premise that two oppositely charged types of toner particles, if mixed in the same bath, tend to electrically neutralize each other, thus producing an electrostatically neutral substance incapable of producing a toner image. As described herein, the present invention is based on the contradictory premise that two oppositely charged types of toner particles mixed in the same bath can coexist, and the mixture will in fact be capable of producing toner images.
In U.S. Pat. No. 4,562,129 (Tanaka), there is described a method of forming monochromatic or dichromatic copy images by use of a developer comprising a high-resistivity magnetic carrier and a nonmagnetic insulating toner which are triboelectrically chargeable, with the result that the toner and carrier adhere to first and second image portions, respectively, of the electrostatic latent image which has at least three levels of potential.
In "Experimental Example 1" of Tanaka, a toner of 14 microns mean particle size and resistivity of at least 10.sup.15 ohm-cm, triboelectrically chargeable to positive polarity, comprised styrene-acrylic copolymer (100) and red colored charge controlling pigment (5); while a carrier of 20 microns mean particle size, resistivity of 10.sup.14 ohm-cm, triboelectrically chargeable to negative polarity, comprised styrene-acrylic copolymer (100), magnetic fine powder (200), carbon black (4) and silica fluidity agent (1.5); parts indicated in parentheses are by weight. The developer comprised a 1:9 mixture of toner and carrier.
The disclosures of U.S. Pat. Nos. 4,068,938, 4,264,185 and 4,562,129 are incorporated by reference herein.
Reference is now made to the following published patent applications and issued patents in the field of electrophotography: GB published Patent Applications Nos. 2,169,416A and 2,176,904A and U.S. Pat. Nos. 3,990,696, 4,233,381, 4,253,656, 4,256,820, 4,269,504, 4,278,884, 4,286,039, 4,302,093, 4,326,644, 4,326,792, 4,334,762, 4,350,333, 4,355,883, 4,362,297, 4,364,460, 4,364,657, 4,364,661, 4,368,881, 4,378,422, 4,392,742, 4,396,187, 4,400,079, 4,411,976, 4,412,383, 4,413,048, 4,418,903, 4,420,244, 4,435,068, 4,439,035, 4,454,215, 4,460,667, 4,473,865, 4,480,825, 4,501,486, 4,522,484, 4,531,824, 4,538,899, 4,582,774, 4,585,329, 4,586,810, 4,589,761, 4,598,992, 4,603,766, 4,620,699, 4,627,705, 4,678,317, the disclosures of which are incorporated by reference herein.